This invention relates to a milling cutter suitable for high speed cutting with a long service life, such as a face mill or end mill for a work-piece of cast iron or steel, and a cutting tool suitable for precision cutting of ferrous materials, and a process for the production of the same.
In the production of an engine or a driving part of a car, or parts used in electric appliances, high speed steel tools, cemented carbide tools, coating tools, ceramic tools or cubic boron nitride sintered compact tools (which will hereinafter be referred to as xe2x80x9ccBN toolxe2x80x9d) have been used for face milling cutters or end mills for cutting cast irons or steels as the materials of the parts.
The cutting speed of a cemented carbide tool or coating tool as the face milling cutter for cutting cast irons is 150 to 250 m/min and the cutting speed practically used in ceramic tools about 400 m/min. On the other hand, in a cBN tool excellent in wear resistance as well as high speed cutting property, a cutting speed of 500 to 1500 m/min is possible by dry process, as proposed in JP-A-8-141822. When a workpiece is markedly subject to influences such as deformation or strain by heat generated during dry process cutting, or when such a part is treated that slight deformation due to heat is considered as a problem, however, cutting must have been effected while decreasing the cutting speed to such an extent that any deformation due to heat does not occur by wet process cutting using a cutting fluid and thermal cracks do not occur at the cutting edge of the cBN tool. That is to say, in the case of wet process cutting, a practically used range for the cutting speed should be 500 to 700 m/min and at a cutting speed exceeding this range, thermal cracks occur at the cutting edge of the tool to remarkably decrease the tool service life. This is due to that in dry process cutting using no cutting fluid, the temperature difference of the heat cycle is so small that the edge part of the cBN tool can resist thermal shock, while in high speed cutting by wet process, the cutting edge at a high temperature during contacting with a work-piece is rapidly cooled during air cutting, so that thermal cracks occur by the heat cycle imparted to the cutting edge.
The practically used cutting speed of a cemented carbide tool or coating tool as a face milling cutter for cutting steels is about 50 to 200 m/min. At a higher speed than this range, the cutting edge encounters rapid wearing or breakage to markedly decrease the tool life. In the case of a cBN tool, cutting is possible at a cutting speed comparable to that of the cemented carbide tool, but the cBN tool has not practically been used as the face milling cutter for cutting steels, since the cBN tool has an equal tool life to the cemented carbide and at a higher speed cutting than this range, the cutting edge encounters breakage due to lowering of the strength of the sintered compact with increase of the temperature of the cutting edge, and occurrence of thermal cracks resulting in marked lowering of the tool life.
The practically used cutting speed of a cemented carbide tool or coating tool as an end mill for cutting cast iron is about 30 to 150 m/min. In a cBN tool, on the other hand, a cutting speed of 100 to 1500 m/min is possible by dry process. By wet process, however, a cutting speed of 100 to 300 m/min is practically used, and at a cutting speed of more than this range, thermal cracks occurs on the cutting edge to markedly lower the tool life in the similar manner to the face milling cutter.
The practically used cutting speed of a cemented carbide tool or coating tool as an end mill for cutting steels is about 30 to 100 m/min. Under a condition of a relatively low cutting speed in a cBN tool, a tool life comparable to that of the cemented carbide tool is only obtained in the similar manner to the face milling cutter, and at a higher speed cutting, the cutting edge of a cBN tool encounters breakage due to lowering of the strength of the sintered compact with increase of the temperature of the cutting edge, and occurrence of thermal cracks resulting in marked lowering of the tool life, so that the cBN tool has not practically been used as the end mill for cutting steels,
The lowering of the service life of a cBN tool under the above described conditions is probably due to the following reasons. A cBN compact of the prior art is obtained by sintering cBN powder grains with a binder such as TiN, TiC, Co, etc. at an ultra-high pressure and contains 10 to 60 volume % of the binder. Since the cBN compact has a thermal conductivity of less than 200 W/mxc2x7K and a thermal expansion coefficient at 20 to 600xc2x0 C. of at least 4.0xc3x9710xe2x88x926/K, it is considered due to that a larger temperature gradient is caused in the vicinity of a cutting edge by the lower thermal conductivity for the temperature difference of a heat cycle during cutting steels or wet process cutting cast irons, a higher tensile stress is caused on the cutting edge during cooling and furthermore, larger extent expansions and shrinkages are repeated to readily cause thermal cracks by the higher thermal expansion coefficient. In addition, it is considered due to that even if the transverse rupture strength as the bending strength at room temperature is at least 80 kgf/mm2, the transverse rupture strength ia rapidly lowered at a temperature of at least 800xc2x0 C.
Therefore, to this end, a tool is required which does not contain any binder at its edge part, does have a high thermal conductivity and low thermal expansion coefficient and does not meet with decrease of the strength even at a high temperature.
On the other hand, lately, requirements for high precision finishing cutting working of high hardness ferrous materials are increasing. For the precision working of the ferrous materials, single crystal diamond and single crystal cubic boron nitride have been investigated.
In the case of cutting a ferrous material by single crystal diamond, however, there arises a problem that a chemical reaction of diamond and iron takes place by cutting heat, resulting in rapid wear of the diamond tool and thus, direct working of a metallic mold of steels, etc. is impossible. Accordingly, in the precision working of a metallic mold for a lens, for example, a method comprising applying an electroless nickel plating layer and precisely finishing the plated layer has been adopted, but this method meet with such a problem that the strength of the metallic mold is not sufficient and the process is complicated. The direct working has been investigated based on a method for suppressing chemical reactions using a special atmosphere, but this is not practical.
Cubic boron nitride (cBN) is a material having a hardness next to diamond and high thermal and chemical stability, whose reactivity with ferrous metals is low. However, a cBN compact having at present been used as a cutting tool is obtained by sintering cBN powder grains with a binder such as TiN, TiC, Co, etc. at an ultra-high pressure and contains 10 to 60 volume % of the binder, as described above. Thus, during shaping the cutting edge, a fine chipping edge tends to occur and it is very difficult to sharply finish the cutting edge without chipping of the edge, so that use thereof as a precision cutting tool be difficult. In order to solve this problem, it is necessary to prepare a tool from a single crystal of cBN or free from a binder. Therefore, a trial for preparing a single crystal of cBN and using as a cutting tool for ultra-precision working of steels has been made, but synthesis of a large-sized cBN single crystal with less impurities and defects is very difficult and a cBN single crystal has a number of cleavage planes, so that the strength is low and the wear resistance is not sufficient. Accordingly, the cBN single crystal has not been put to practical use up to the present time.
As apparent from the foregoing illustrations, it is considered that both of a cutting tool for subjecting cast irons or steels to high speed milling working and a cutting tool suitable for precision cutting of ferrous materials can be realized by a cBN compact tool free from binders.
As the cBN compact tool free from binders, there is a compact obtained by subjecting hexagonal boron nitride (which will hereinafter be referred to as hBN) as a raw material to reaction sintering using magnesium boronitride, etc., as a catalyst. This compact comprises cBN grains strongly bonded with each other, being free from a binder and having a thermal conductivity of 600 to 700 W/mxc2x7K, which is applied to heat sink materials or TAB bonding tools. Since there remain some catalyst in this compact, however, fine cracks tend to occur due to difference between catalyst and cBN in thermal expansion when heat is added. Thus, the heat resistance temperature is low, i.e. about 700xc2x0 C., resulting in a large problem as a cutting tool. Moreover, the grain diameter is so large as represented by approximately 10 xcexcm that the strength is not sufficient although the thermal conductivity is high and the compact cannot be applied to a cutting tool.
On the other hand, cBN can be synthesized (directly converted) at an ultra-high pressure and high temperature from BN of normal pressure type, e.g hBN without using any catalyst. It is known that a binder-free cBN sintered compact can be produced by direct conversion of hBNxe2x86x92cBN with simultaneous sintering.
For example, JP-A-47-34099 and JP-A-3-159964 disclose a method comprising converting hBN into cBN at an ultra-high pressure and high temperature and thus obtaining a cBN sintered compact. In addition, JP-B-63-394 and JP-A-8-47801 disclose a method comprising preparing cBN from pyrolytic boron nitride (which will hereinafter be referred to as pBN). However, these cBN sintered compacts have the problems that compressed hBN crystal grains at an ultra-high pressure tends to remain in the cBN sintered compact and exhibit strong orientation property (anisotropic property) of cBN crystals, resulting in laminar cracking or stripping.
Furthermore, as a method of obtaining cBN by direct conversion, for example, JP-B-49-27518 discloses using hexagonal boron nitride, as a raw material, having an average grain diameter of primary grains in a range of at most 3 xcexcm. However, the thus obtained cBN cannot be applied to a cutting tool, because hexagonal boron nitride in the form of fine powder contains several %, of boron oxide impurity and adsorbed gases, so that sintering does not sufficiently proceed, and the sintered compact contains such a large amount of the oxide as not giving high hardness, high strength and excellent heat resistance.
Since the cBN sintered compact of the prior art, containing a binder, has a low thermal conductivity and large thermal expansion coefficient and tends to encounter thermal cracks by a large load of the heat cycle and further, the strength is decreased at a high temperature, cutting of cast irons by wet process or high speed milling of steels is impossible. In addition, no sharp cutting edge can be obtained, the strength or wear resistance of the cutting edge is not sufficient and accordingly, precision cutting working of ferrous materials is impossible. As to a cBN single crystal, synthesis of a large sized cBN single crystal with less impurities and defects is very difficult and its strength is low and wear resistance is not sufficient. Thus, cBN encounters breakage of the cutting edge or wearing by microchipping through cleavage of (110) plane or (111) plane.
If a sintered compact of cBN single phase free from a binder, in which structure grains are fine and strongly bonded with each other, can be obtained by direct conversion, it is considered that the sintered compact has a high thermal conductivity and small thermal expansion coefficient and the transverse rupture strength is not lowered even at a high temperature, so that breakage due to thermal cracks can be suppressed and high speed milling of cast irons by wet process or high speed milling of steels is rendered possible. Furthermore, a sharp cutting edge can be formed by fine granulation of cBN, the edge breakage or wearing due to cleavage can be improved and the precision machining of ferrous materials can be carried out.
In the binder-free cBN sintered compact of the prior art, however, as described above, the grain diameter is large, i.e. several xcexcm and further, there are present the catalyst, compressed hBN, oxides, etc. in the grain boundaries to lower the transverse rupture strength and heat resistance temperature, so that the tool edge does not have an edge strength required for milling cutting being at a high temperature and a sharp cutting edge required for a precision cutting tool cannot be obtained. In the direct conversion method of the prior art, hBN as a raw material tends to be orientated and to become a sintered compact orientated in (111) direction. When pBN originally having a high orientation is used as a raw material, there is obtained a cBN sintered compact more orientated in (111) direction than in the case of using hBN as a raw material. Because of this orientation, there arises a problem that when using it as a cutting tool, disadvantages such as laminar cracks or strippings take place. A sintered compact of cBN single phase being isotropic and finely granular and having such a high bonding strength among grains as to be applied to cutting use has not been known up to the present time.
In the case of carrying out cutting of cast irons by wet process using the cBN tool of the prior art, therefore increasing the cutting speed so as to be similar to cutting by dry process results in decrease of the tool life in any of the face milling or end mill, consequently, raising the production cost.
For milling steels, inrease of the cutting speed using a cBN tool results in only lowering of the tool life and even at the generally employed working speed of a cemented carbide tool, the use of an expensive cBN tool having a similar service life to a cemented carbide tool raises the cost of cutting process. This is not preferable.
However, in recent years, mechanical working machines capable of high speed rotation have been developed one after another and high speed cutting is indispensable in order to improve the working efficency and reduce the cost. In such a mechanical working machine, it is eagerly desired to provide a cutting tool capable of corresponding to high speed cutting of steels and wet process cutting so as to suppress influences upon a workpiece due to rising of the cutting temperature during cutting a part of cast iron.
The inventors have made efforts to solve various problems of the prior art, as described above, in milling cutters and precision cutting tools such as cBN tools, in particular, face milling cutters or end mills and consequently, have reached the present invention.
The principal object is to provide a milling cutter using cBN, being free from binders and having a grain size of at most 1 xcexcm, and having a high thermal conductivity and small thermal expansion coefficient and being excellent in strength as well as wear resistance, because of containing no impurity in the grain boundaries and having an isotropic structure, whereby to exhibit high speed cutting of a cutting speed of at least 800 m/min, preferably at least 1000 m/min in the case of a face mill cutter for cutting cast iron by wet process; high speed cutting of a cutting speed of at least 300 m/min, preferably at least 500 m/min by wet process in the case of an end mill; a cutting speed of at least 200 m/min in the case of a face mill cutter for cutting steels by dry process and wet process; and a cutting speed of at least 150 m/min in the case of an end mill for cutting steels by dry process and by wet process, and whereby to achieve a sufficient tool life.
The second object is to provide a precision cutting tool consisting of fine grain cBN free from binders and having a grain size of at most 0.5 xcexcm, and having a very sharp cutting edge, because of containing no impurity in the grain boundary and having an isotropic structure.
The present invention is constructed of the following summarized inventions and embodiments.
(1) A cutting tool compring, as an edge part, a cubic boron nitride sintered compact containing cubic boron nitride having an average grain diameter of at most 1 xcexcm, in particular, at most 0.5 xcexcm, in which the cubic boron nitride sintered compact has, at the said edge part, an I(22O)/I(111) of (220) diffraction intensity (I(220)) to (111) diffraction intensity (I(111)) ratio of at least 0.05, in particular, at least 0.1 in X-ray diffraction of arbitrary direction and impurities are substantially not contained in the grain boundaries.
(2) The cutting tool as described in the above (1), wherein the thermal conductivity of the cubic boron nitride sintered compact, at the said edge part, is 250 to 1000 W/mxc2x7K.
(3) The cutting tool as described in the above (1) or (2), wherein the transverse rupture strength of the said cubic boron nitride sintered compact is at least 80 kgf/mm2 by a three point bending measurement at a temperature between 20xc2x0 C. and 1000xc2x0 C.
(4) The cutting tool as described in any one of the above (1) to (3), wherein the hardness of the cubic boron nitride sintered compact, at the said edge part, is at least 4000 kgf/mm2 at room temperature.
(5) The milling cutter as described in any one of the above (1) to (4), wherein the thermal conductivity of the cubic boron nitride sintered compact, at the said edge part, is 300 to 1000 W/mxc2x7K.
(6) The milling cutter as described in any one of the above (1) to (5), wherein the thermal expansion coefficient of the cubic boron nitride sintered compact, at the said edge part, is 3.0 to 4.0xc3x9710xe2x88x926/K at a temperature ranging from 20xc2x0 C. to 600xc2x0 C.
(7) The milling cutter as described in any one of the above (1) to (6), which is applied to a face milling cutter or end mill for high speed cutting cast irons or steels.
(8) The precision cutting tool as described in any one of the above (1) to (4), wherein the cubic boron nitride sintered compact, at the said edge part, contains cBN with an average grain diameter of at most 0.5 xcexcm.
(9) A process for the production of a sintered compact for a cutting tool containing cubic boron nitride with an average grain diameter of at most 1 xcexcm, in particular, at most 0.5 xcexcm, which comprises reducing and nitriding a compound containing boron and oxygen in the presence of carbon and nitrogen to synthesize a low pressure phase boron nitride and subjecting the resulting low pressure phase boron nitride, as a starting material, to direct conversion into cubic boron nitride at a high temperature and high pressure, while simultaneously sintering.
(10) The process for the production of a sintered compact for a cutting tool, as described in the above (9), wherein the said direct conversion and sintering are carried out at a pressure of at least 6 GPa and a temperature of 1550 to 2100xc2x0 C.
As described above, a cBN sintered compact for composing the cutting tool of the present invention contains no binder, comprises cBN of at most 1 xcexcm, contains no impurities at grain boundaries and has an isotropic structure, which can favorably be used as a milling cutter for high speed machining, having a high thermal conductivity and low thermal expansion coefficient and being excellent in strength as well as wear resistance. If cBN is of fine grains with a grain size of at most 0.5 xcexcm, there can be obtained a tool having a sharp cutting edge and higher strength and wear resistance, which can preferably be applied to a precision cutting use.
The cBN sintered compact of the cutting tool according to the present invention is obtained by subjecting low crystallinity BN or fine grain, normal pressure type BN free from adsorbed gases or boron oxide, as a starting material, to direct conversion into cBN, followed by sintering, at a high pressure and high temperature. It is required that the low crystallinity BN or fine grain, normal pressure type BN used herein is prepared by reducing boron oxide or boric acid with carbon, followed by nitriding. As a method of synthesizing BN of normal pressure type, it is generally and industrially carried out to react boron oxide or boric acid with ammonia. However, when the thus obtained BN is heat treated at a high temperature, it is crystallized into hBN. Thus, even if fine grain, low crystallinity BN of normal pressure type is synthesized by this method, it is crystallized into hBN and subject to grain growth by a high temperature puirification treatment to remove boron oxide as an impurity (at least 2050xc2x0 C. in nitrigen gas, at least 1650xc2x0 C. in vacuum). In contrast, the normal pressure type BN prepared by reducing boron oxide or boric acid with carbon, followed by nitriding, has such a feature that it is not crystallized even if heat-treated at a high temperature. Accordingly, the boron oxide- or adsorbed gases-free normal pressure type BN suitable for direct conversion and sintering can be obtained by synthesizing the fine grain and low crystallinity normal pressure type BN by this method and subjecting it to a high purity purification treatment in a nitrogen atmosphere at 2050xc2x0 C. or higher or in vacuum at 1650xc2x0 C. or higher. The above described reducing and nitriding can be carried out using nitrogen and carbon as a heating source.
Since according to the present invention, the starting material is the fine grain and atmospheric pressure type BN and does not contain boron oxide hindering the cBN conversion, compressed hBN often appearing in the prior art direct conversion method does not remain and there is less grain growth or less monoaxial orientation in the cBN after the direct conversion. Consequently, an isotropic sintered compact consisting of fine grains is obtained, whose bonding strength among the grains is higher because of absence of boron oxide hindering sintering of cBN grains with each other.
A preferable condition for the above described direct conversion is a pressure of at least 6 GPa and a temperature of 1550 to 2100xc2x0 C. In particular, the sintering temperature is more important, since if lower than the the range, the conversion into cBN is not sufficient, while if higher than it, grain growth of cBN proceeds to decrease the bonding strength of cBN grains with each other. The sintering temperature, at which grain growth of cBN does not occur, varies with the crystallinity and grain diameter of the starting material. The cBN sintered compact, sintered in the above described suitable sintering temperature range, has such a feature that the compact has a dense structure comprising cBN with an average grain diameter of at most 1.0 xcexcm, preferably 0.5 xcexcm, contains no impurity in the grain boundaries, and has an isotropic structure.
Control of the grain diameter of cBN is generally carried out at a temperature during the direct conversion. That is, in order to control the fine grain state of at most 1.0 xcexcm, it is necessary to use the fine grain and low crystallinity normal pressure type BN and subject it to direct conversion at a low temperature range. When using ordinary hBN or pBN, the conversion into cBN does not occur unless the temperature is raised to 2100xc2x0 C. or higher and accordingly, it is impossible to control to a grain size of at most 1.0 xcexcm.
When the cBN sintered compact obtained in this way is applied to a material for a milling cutter or precision cutting tool, there is obtained a very sharp and high strength cutting edge having a high thermal conductivity and low thermal expansion coefficient and being rendered possible high speed milling and precision cutting, which have been considered difficult in the prior art.
The cutting tool according to the present invention has a cutting edge consisting of a sintered compact comprising a cubic boron nitride (cBN) having an average grain diameter of at most 1.0 xcexcm, obtained by subjecting a low pressure phase boron nitride to direct conversion at a high pressure and high temperature and simultaneously, to sintering, the cBN sintered compact having an I(220)/I(111) of (220) diffraction intensity (I(220)) to (111) diffraction intensity (I(111)) ratio of at least 0.05, in particular, at least 0.1 in X-ray diffraction of arbitrary direction and impurities subtantially not contained in the grain boundaries. When the average grain diameter of cBN exceeds 1 xcexcm, an edge strength required for milling cannot be obtained. When the average grain diameter of cBN exceeds 0.5 xcexcm, a cutting edge has not a sharpness sufficient for precision cutting and the strength is not sufficient. If the X-ray diffraction intensity ratio, I(220)/I(111) is less than 0.05, the cBN sintered compact exhibits a strong orientation in  less than 111 greater than  direction, i.e. being anisotropic, so that laminar cracks or strippings tend to occur.
Preferably, the edge part of the cBN sintered compact has a transverse rupture strength of at least 80 kgf/mm2, whose strength is not lowered even at a high temperature. If less than 80 kgf/mm2 or the strength is lowered at a high temperature, a cuttinn edge having a sufficient strength cannot be obtained and breakage tends to occur. The hardness of the cBN sintered compact, at an edge part, is preferably at least 4000 kgf/mm2. If less than 4000 kgf/mm2, wearing during cutting is large, the service life is shortened and a precision cutting cannot be carried out.
The edge part of the cBN sintered compact preferably has a thermal conductivity of at least 250 W/mxc2x7K and a thermal expansion coefficient of at most 4.0xc3x9710xe2x88x926/K. If the thermal conductivity is less than 250 W/mxc2x7K, a large temperature gradient is formed during cutting in the vicinity of the cutting edge to cause a high tensile stress at the cutting edge during cooling, whileif the thermal expansion coefficient is more than 4.0xc3x9710xe2x88x926/K, the cutting edge repeatedly expands and shrinks by the heat cycle during cutting, thus forming thermal cracks at the edbe part and markedly shortening the service life of the tool.
As illustrated above, the cBN sintered compact for the cutting tool of the present invention contains no binder, contains no impurities at grain boundaries and has an isotropic structure, so there can be obtained a cutting edge having a high thermal conductivity and low thermal expansion coefficient and being excellent in heat resistance cracking property and strength, whereby high speed milling is rendered possible. Thus, a longer serveice life than that of the prior art tools can be given when using as a cutting tool for wet process cutting of cast irons or high speed milling of steels. Furthermore, a sharp cutting edge capable of effecting precision working and being excellent in strength as well as wear resistance can be obtained by composing the cutting edge of fine grain cBN. When using the cutting tool of the present invention for precision working of ferrous materials, therefore, excellent properties can be realized which cannot be found in the prior art sintered compact.